This invention relates to structures for and methods of manufacture of power transistors, and more specifically relates to an increased concentration region between closely spaced base or body regions which is deeper than the bases. A very deep increased concentration region in an insulated gate bipolar transistor (hereinafter an "IGBT") permits a substantial increase in the latch current of the device and a substantial decrease in switching loss without increasing forward voltage drop and without significantly reducing breakdown voltage. When applied to either an IGBT or power MOSFET, the invention also forces a uniform avalanche breakdown from the bottoms of the deep increased concentration regions to improve the device I.sup.2 L capability. The very deep increased concentration region also permits the use of heavy metal diffusion to increase lifetime killing and reduce switching loss without an excessive increase in forward voltage drop.
Power IGBT devices employ the low current gate control of a power MOSFET device, which is capable of very high switching speed in combination with a bipolar type of device which operates with a high current density. By a "power device" is meant a device having the capability of controlling more than approximately 1 watt, and is distinguished from the signal processing type of device which handles much smaller power levels. IGBT devices, while slower than a standard power MOSFET, are still much faster than comparable power bipolar transistors, are voltage controlled and have significantly higher current densities than comparable power MOSFETs.
IGBT devices are shown, for example, in U.S. Pat. No. 4,672,407 dated Jun. 9, 1987 and U.S. Pat. No. 4,364,073 dated Dec. 14, 1982.
A properly designed IGBT employs design principles which ordinarily would be used for a low voltage power MOSFET geometry, particularly very small line widths for the poly gate. However, the IGBT is used primarily at 500 volts and above so that the designer must use the low voltage design on a high voltage starting material which has a high resistivity relatively thick epitaxial material. A high resistivity epitaxial material must be used for receiving the junction patterns since the higher the resistivity of the material, the higher the voltage blocking capability of the device. This higher resistivity material ordinarily increases on-resistance.
Existing IGBT devices have switching frequencies less than about 25 kHz due primarily to a long fall time of collector current during turn-off. These long fall times produce high forward conduction switching losses, requiring increased silicon chip area to meet a given current rating. One way to reduce such forward conduction switching losses is to increase the space between base or body regions forming the device junction pattern. Increasing the space between bases leads to a poor packing density and an inefficient use of the silicon surface, and makes the device less immune to latch-up of the inherent parasitic thyristor present in the junction pattern. It would be desirable to reduce losses while keeping a small spacing between device cells.
The following terminology shall be used hereinafter to identify the electrode and functions of an N channel IGBT:
The emitter terminal of the packaged unit is connected to the front side power electrode of the die. It is sometimes called the cathode terminal and, in a power MOSFET, is the source terminal.
The collector terminal of the packaged unit is connected to the back side power electrode of the die. It is sometimes called the anode terminal and, in a power MOSFET, is the drain terminal. It is also the emitter of the internal PNP transistor.
The P-type base region of the MOSFET in the IGBT is sometimes called the body region. It is the base of the internal NPN transistor and is also the internal collector of the PNP transistor.
In general in an IGBT, the smaller the space between bases, the larger the latch current. More specifically, when an N channel IGBT operates in its forward conduction mode, carriers are injected across the back side emitter-base junction and toward the front side emitter electrode. If there is a large space between bases, a larger percentage of the full collector current flows into the side walls of the surface base regions and under the front side emitter and through the resistance R.sub.b ' beneath the front side emitter. This can then latch the parasitic thyristor at a lower current. A smaller space between bases reduces this effect. However, if there is a small space between bases, there will be almost no conductivity modulation in that area since holes are swept away by collection at the bottom of the deep base junction before they can modulate the active region. Also, decreasing the space between the bases and increasing the length of the vertical conduction path between closely spaced bases also increases the pinch effect of the parasitic JFET defined between the bases. In a power MOSFET, this causes a substantial increase in device on-resistance, and in an IGBT it causes a substantial increase in the forward voltage drop. It would be desirable to have a small space between bases for a large latch current while also having a low forward voltage drop.
It is known that the efficiency of the parasitic JFET can be reduced by increasing the conductivity in the space between the bases of the MOSFET portion of the device. This is sometimes called an enhancement diffusion or increased conductivity region. Such increased conductivity regions for power MOSFETs are shown in U.S. Pat. Nos. 4,376,286 and 4,593,302, each of which is owned by the assignee of the present invention. Such increased conductivity regions are employed in the power MOSFET products sold by the assignee of the application under its registered trademark "HEXFET". In practice, the implant dose used to disable the parasitic JFET of a power MOSFET is about 1.times.10.sup.12 atoms/cm.sup.2. Higher doses will begin to degrade the reverse breakdown voltage of the MOSFET. This same kind of increased concentration region has also been used in prior art IGBT devices sold by the assignee of the present invention, for example, its IGBT part numbers IRGBC20, IRGBC30, IRGBC40, IRGPC40 and IRGPC50. These IGBTs use an implant dose of 3.5.times.10.sup.12 atoms per cm.sup.2 which is diffused to a depth deeper than the source, but shallower than the deep base. This enhancement diffusion increases latch current since it permits closer packing of the cells and, thus a smaller poly line width. However, this enhancement diffusion, due to its depth, does not offset the parasitic JFET over its full length.
Typically, the depth of the increased conductivity region in prior art IGBTs is about 3 microns while the deep base was about 6 microns. Furthermore, in these prior art IGBT devices, as manufacturing tolerances improved, the source region became smaller in lateral extent and the deep base larger and the shape of the base or body became more squared in cross-section. Thus, the effective length of the JFET between bases increased in length. However, the increased concentration region remained at about 3 microns in depth and extended for only about one half the length of the effective parasitic JFET produced between spaced bases. It would be desirable to be able to defeat the parasitic JFET over its full effective length.
In some cases, it may be desirable not to use any lifetime killing in an IGBT. However, the switching speed of an IGBT may be increased by reducing the lifetime of the carriers in the silicon. In prior art IGBTs sold by the assignee of the invention, lifetime was reduced by electron beam irradiation of the completed chip by a dose of about 8 megarads. This produced, in one particular device, a fall time of about 300 nanoseconds and a turn-off switching loss of about 600 microjoules. However, reduction of lifetime in an IGBT increases forward voltage drop since it reduces the gain of the bipolar transistor portion of the device. That is, there is less conductivity modulation for the same gate voltage in the presence of reduced gain. It would be desirable to lower switching losses by using a higher radiation dose without increasing forward voltage drop.
Electron irradiation is used in the assignee's prior art IGBTs instead of heavy metal doping, e.g. gold or platinum, because heavy metal doping increases the apparent resistivity in the active region between bases, thus further increasing the JFET pinch between bases. However, the effect of radiation can anneal out at die bond temperatures, which complicates the assembly process. Therefore, in many cases, heavy metal lifetime killing is preferred to radiation. It would be desirable to be able to use heavy metal doping in an IGBT without increasing forward voltage drop above that of a comparable electron irradiated IGBT.
An important characteristic of power MOSFETs and IGBTs is their avalanche energy. Generally, avalanche occurs at relatively few sites at the periphery of the device. Therefore, in the IGBT, the emitter base junction of the active bipolar transistor is non-uniformly biased and injects non-uniformly in small areas with high current density, leading to local failure. It would be desirable to improve the avalanche energy of a power MOSFET or IGBT.
From the above, it will be seen that the use of a narrower poly line width, that is, closely spaced bases, has the benefit of increasing latch current and the device current density, but the drawback of causing higher forward voltage drop. Lifetime killing can be used to increase switching speed at the expense of increased forward voltage drop.